Food Perception Primes Hepatic ER Homeostasis via Melanocortin-Dependent Control of mTOR Activation.

Adaptation of liver to the postprandial state requires coordinated regulation of protein synthesis and folding aligned with changes in lipid metabolism. Here we demonstrate that sensory food perception is sufficient to elicit early activation of hepatic mTOR signaling, Xbp1 splicing, increased expression of ER-stress genes, and phosphatidylcholine synthesis, which translate into a rapid morphological ER remodeling. These responses overlap with those activated during refeeding, where they are maintained and constantly increased upon nutrient supply. Sensory food perception activates POMC neurons in the hypothalamus, optogenetic activation of POMC neurons activates hepatic mTOR signaling and Xbp1 splicing, whereas lack of MC4R expression attenuates these responses to sensory food perception. Chemogenetic POMC-neuron activation promotes sympathetic nerve activity (SNA) subserving the liver, and norepinephrine evokes the same responses in hepatocytes in vitro and in liver in vivo as observed upon sensory food perception. Collectively, our experiments unravel that sensory food perception coordinately primes postprandial liver ER adaption through a melanocortin-SNA-mTOR-Xbp1s axis. VIDEO ABSTRACT.

AgRP Neurons Control Systemic Insulin Sensitivity via Myostatin Expression in Brown Adipose Tissue.

Activation of Agouti-related peptide (AgRP) neurons potently promotes feeding, and chronically altering their activity also affects peripheral glucose homeostasis. We demonstrate that acute activation of AgRP neurons causes insulin resistance through impairment of insulin-stimulated glucose uptake into brown adipose tissue (BAT). AgRP neuron activation acutely reprograms gene expression in BAT toward a myogenic signature, including increased expression of myostatin. Interference with myostatin activity improves insulin sensitivity that was impaired by AgRP neurons activation. Optogenetic circuitry mapping reveals that feeding and insulin sensitivity are controlled by both distinct and overlapping projections. Stimulation of AgRP → LHA projections impairs insulin sensitivity and promotes feeding while activation of AgRP → anterior bed nucleus of the stria terminalis (aBNST)vl projections, distinct from AgRP → aBNSTdm projections controlling feeding, mediate the effect of AgRP neuron activation on BAT-myostatin expression and insulin sensitivity. Collectively, our results suggest that AgRP neurons in mice induce not only eating, but also insulin resistance by stimulating expression of muscle-related genes in BAT, revealing a mechanism by which these neurons rapidly coordinate hunger states with glucose homeostasis.

Melanocortin-4 receptors on neurons in the parabrachial nucleus mediate inflammation-induced suppression of food-seeking behavior.

Anorexia is a common symptom during infectious and inflammatory disease. Here we examined the role of melanocortin-4 receptors (MC4Rs) in inflammation-induced anorexia. Mice with transcriptional blockage of the MC4Rs displayed the same reduction of food intake following peripheral injection of lipopolysaccharide as wild type mice but were protected against the anorexic effect of the immune challenge in a test in which fasted animals were to use olfactory cues to find a hidden cookie. By using selective virus-mediated receptor re-expression we demonstrate that the suppression of the food-seeking behavior is subserved by MC4Rs in the brain stem parabrachial nucleus, a central hub for interoceptive information involved in the regulation of food intake. Furthermore, the selective expression of MC4R in the parabrachial nucleus also attenuated the body weight increase that characterizes MC4R KO mice. These data extend on the functions of the MC4Rs and show that MC4Rs in the parabrachial nucleus are critically involved in the anorexic response to peripheral inflammation but also contribute to body weight homeostasis during normal conditions.

Glucagon-Like Peptide-1-, but not Growth and Differentiation Factor 15-, Receptor Activation Increases the Number of Interleukin-6-Expressing Cells in the External Lateral Parabrachial Nucleus.

Interleukin (IL)-6 in the hypothalamus and hindbrain is an important downstream mediator of suppression of body weight and food intake by glucagon-like peptide-1 (GLP-1) receptor stimulation. CNS GLP-1 is produced almost exclusively in prepro-glucagon neurons in the nucleus of the solitary tract. These neurons innervate energy balance-regulating areas, such as the external lateral parabrachial nucleus (PBNel); essential for induction of anorexia. Using a validated novel IL-6-reporter mouse strain, we investigated the interactions in PBNel between GLP-1, IL-6, and calcitonin gene-related peptide (CGRP, a well-known mediator of anorexia). We show that PBNel GLP-1R-containing cells highly (to about 80%) overlap with IL-6-containing cells on both protein and mRNA level. Intraperitoneal administration of a GLP-1 analogue exendin-4 to mice increased the proportion of IL-6-containing cells in PBNel 3-fold, while there was no effect in the rest of the lateral parabrachial nucleus. In contrast, injections of an anorexigenic peptide growth and differentiation factor 15 (GDF15) markedly increased the proportion of CGRP-containing cells, while IL-6-containing cells were not affected. In summary, GLP-1R are found on IL-6-producing cells in PBNel, and GLP-1R stimulation leads to an increase in the proportion of cells with IL-6-reporter fluorescence, supporting IL-6 mediation of GLP-1 effects on energy balance.

Activation of GFRAL+ neurons induces hypothermia and glucoregulatory responses associated with nausea and torpor.

GFRAL-expressing neurons actuate aversion and nausea, are targets for obesity treatment, and may mediate metformin effects by long-term GDF15-GFRAL agonism. Whether GFRAL+ neurons acutely regulate glucose and energy homeostasis is, however, underexplored. Here, we report that cell-specific activation of GFRAL+ neurons using a variety of techniques causes a torpor-like state, including hypothermia, the release of stress hormones, a shift from glucose to lipid oxidation, and impaired insulin sensitivity, glucose tolerance, and skeletal muscle glucose uptake but augmented glucose uptake in visceral fat. Metabolomic analysis of blood and transcriptomics of muscle and fat indicate alterations in ketogenesis, insulin signaling, adipose tissue differentiation and mitogenesis, and energy fluxes. Our findings indicate that acute GFRAL+ neuron activation induces endocrine and gluco- and thermoregulatory responses associated with nausea and torpor. While chronic activation of GFRAL signaling promotes weight loss in obesity, these results show that acute activation of GFRAL+ neurons causes hypothermia and hyperglycemia.

Cancer-induced anorexia in tumor-bearing mice is dependent on cyclooxygenase-1.

It is well-established that prostaglandins (PGs) affect tumorigenesis, and evidence indicates that PGs also are important for the reduced food intake and body weight loss, the anorexia-cachexia syndrome, in malignant cancer. However, the identity of the PGs and the PG producing cyclooxygenase (COX) species responsible for cancer anorexia-cachexia is unknown. Here, we addressed this issue by transplanting mice with a tumor that elicits anorexia. Meal pattern analysis revealed that the anorexia in the tumor-bearing mice was due to decreased meal frequency. Treatment with a non-selective COX inhibitor attenuated the anorexia, and also tumor growth. When given at manifest anorexia, non-selective COX-inhibitors restored appetite and prevented body weight loss without affecting tumor size. Despite COX-2 induction in the cerebral blood vessels of tumor-bearing mice, a selective COX-2 inhibitor had no effect on the anorexia, whereas selective COX-1 inhibition delayed its onset. Tumor growth was associated with robust increase of PGE(2) levels in plasma - a response blocked both by non-selective COX-inhibition and by selective COX-1 inhibition, but not by COX-2 inhibition. However, there was no increase in PGE(2)-levels in the cerebrospinal fluid. Neutralization of plasma PGE(2) with specific antibodies did not ameliorate the anorexia, and genetic deletion of microsomal PGE synthase-1 (mPGES-1) affected neither anorexia nor tumor growth. Furthermore, tumor-bearing mice lacking EP(4) receptors selectively in the nervous system developed anorexia. These observations suggest that COX-enzymes, most likely COX-1, are involved in cancer-elicited anorexia and weight loss, but that these phenomena occur independently of host mPGES-1, PGE(2) and neuronal EP(4) signaling.

NPY mediates the rapid feeding and glucose metabolism regulatory functions of AgRP neurons.

Activation of Agouti-Related Peptide (AgRP)-expressing neurons promotes feeding and insulin resistance. Here, we examine the contribution of neuropeptide Y (NPY)-dependent signaling to the diverse physiological consequences of activating AgRP neurons. NPY-deficient mice fail to rapidly increase food intake during the first hour of either chemo- or optogenetic activation of AgRP neurons, while the delayed increase in feeding is comparable between control and NPY-deficient mice. Acutely stimulating AgRP neurons fails to induce systemic insulin resistance in NPY-deficient mice, while increased locomotor activity upon AgRP neuron stimulation in the absence of food remains unaffected in these animals. Selective re-expression of NPY in AgRP neurons attenuates the reduced feeding response and reverses the protection from insulin resistance upon optogenetic activation of AgRP neurons in NPY-deficient mice. Collectively, these experiments reveal a pivotal role of NPY-dependent signaling in mediating the rapid feeding inducing effect and the acute glucose regulatory function governed by AgRP neurons.

The Fat Mass and Obesity-Associated Protein (FTO) Regulates Locomotor Responses to Novelty via D2R Medium Spiny Neurons.

Variations in the human FTO gene have been linked to obesity and altered connectivity of the dopaminergic neurocircuitry. Here, we report that fat mass and obesity-associated protein (FTO) in dopamine D2 receptor-expressing medium spiny neurons (D2 MSNs) of mice regulate the excitability of these cells and control their striatopallidal globus pallidus external (GPe) projections. Lack of FTO in D2 MSNs translates into increased locomotor activity to novelty, associated with altered timing behavior, without impairing the ability to control actions or affecting reward-driven and conditioned behavior. Pharmacological manipulations of dopamine D1 receptor (D1R)- or D2R-dependent pathways in these animals reveal altered responses to D1- and D2-MSN-mediated control of motor output. These findings reveal a critical role for FTO to control D2 MSN excitability, their projections to the GPe, and behavioral responses to novelty.

Inhibition of P2Y6 Signaling in AgRP Neurons Reduces Food Intake and Improves Systemic Insulin Sensitivity in Obesity.

Uridine-diphosphate (UDP) and its receptor P2Y6 have recently been identified as regulators of AgRP neurons. UDP promotes feeding via activation of P2Y6 receptors on AgRP neurons, and hypothalamic UDP concentrations are increased in obesity. However, it remained unresolved whether inhibition of P2Y6 signaling pharmacologically, globally, or restricted to AgRP neurons can improve obesity-associated metabolic dysfunctions. Here, we demonstrate that central injection of UDP acutely promotes feeding in diet-induced obese mice and that acute pharmacological blocking of CNS P2Y6 receptors reduces food intake. Importantly, mice with AgRP-neuron-restricted inactivation of P2Y6 exhibit reduced food intake and fat mass as well as improved systemic insulin sensitivity with improved insulin action in liver. Our results reveal that P2Y6 signaling in AgRP neurons is involved in the onset of obesity-associated hyperphagia and systemic insulin resistance. Collectively, these experiments define P2Y6 as a potential target to pharmacologically restrict both feeding and systemic insulin resistance in obesity.

IL-6 Improves Energy and Glucose Homeostasis in Obesity via Enhanced Central IL-6 trans-Signaling.

Interleukin (IL)-6 engages similar signaling mechanisms to leptin. Here, we find that central application of IL-6 in mice suppresses feeding and improves glucose tolerance. In contrast to leptin, whose action is attenuated in obesity, the ability of IL-6 to suppress feeding is enhanced in obese mice. IL-6 suppresses feeding in the absence of neuronal IL-6-receptor (IL-6R) expression in hypothalamic or all forebrain neurons of mice. Conversely, obese mice exhibit increased soluble IL-6R levels in the cerebrospinal fluid. Blocking IL-6 trans-signaling in the CNS abrogates the ability of IL-6 to suppress feeding. Furthermore, gp130 expression is enhanced in the paraventricular nucleus of the hypothalamus (PVH) of obese mice, and deletion of gp130 in the PVH attenuates the beneficial central IL-6 effects on metabolism. Collectively, these experiments indicate that IL-6 trans-signaling is enhanced in the CNS of obese mice, allowing IL-6 to exert its beneficial metabolic effects even under conditions of leptin resistance.

Acetaminophen reduces lipopolysaccharide-induced fever by inhibiting cyclooxygenase-2.

Acetaminophen is one of the world's most commonly used drugs to treat fever and pain, yet its mechanism of action has remained unclear. Here we tested the hypothesis that acetaminophen blocks fever through inhibition of cyclooxygenase-2 (Cox-2), by monitoring lipopolysaccharide induced fever in mice with genetic manipulations of enzymes in the prostaglandin cascade. We exploited the fact that lowered levels of a specific enzyme make the system more sensitive to any further inhibition of the same enzyme. Mice were immune challenged by an intraperitoneal injection of bacterial wall lipopolysaccharide and their body temperature recorded by telemetry. We found that mice heterozygous for Cox-2, but not for microsomal prostaglandin E synthase-1 (mPGES-1), displayed attenuated fever, indicating a rate limiting role of Cox-2. We then titrated a dose of acetaminophen that did not inhibit the lipopolysaccharide-induced fever in wild-type mice. However, when the same dose of acetaminophen was given to Cox-2 heterozygous mice, the febrile response to lipopolysaccharide was strongly attenuated, resulting in an almost normalized temperature curve, whereas no difference was seen between wild-type and heterozygous mPGES-1 mice. Furthermore, the fever to intracerebrally injected prostaglandin E2 was unaffected by acetaminophen treatment. These findings reveal that acetaminophen, similar to aspirin and other non-steroidal anti-inflammatory drugs, is antipyretic by inhibiting cyclooxygenase-2, and not by inhibiting mPGES-1 or signaling cascades downstream of prostaglandin E2.